FYI: Wait for scale change

This change alters the behavior of POS when waiting for the scale to register on a by-weight item. The UPC stores the current scan in the session variable "SNR" rather than just setting that variable to one (I have no idea what the acronym stands for; it's been around forever). When the scale weight is rendered, the UPC is read back via "SNR" and immediately entered as the next item. 

The upside is strictly performance. Instead of having two separate javascript loops - one polling the scale for a weight and one continually retrying the UPC - there's one loop that does both. The two actions are tightly linked.

I'm already using this in production.